The association of the human papovavirus, JCV, with a demyelinating disease described as progressive multifocal leucoencephalopathy (PML) has been firmly established from isolation of virions from PML brain tissue and detection of viral antigen in PML plaque lesions. Seroepidemiological studies have established that the infecting virus, JCV, is widespread in populations throughout the world even though PML is a rare disease, suggesting that JCV establishes a latent or persistent infection. We have undertaken the study of the molecular pathology of JCV and its interaction with human glial cells in culture and as a potential cause of demyelination or tumor production in simians. Our experiments are focused at the intracellular level, designed to assay the molecular nature of JCV infection, and its effect on its host cells as a model for viral persistence. Our current findings now suggest that JCV induced owl monkey glioblastomas when explanted in culture lose the integrated JCV genome from the cell chromosome resulting in loss of the viral T protein and tumor cell phenotypes. We have shown that the JCV T protein is necessary for such glioblastoma cells to continue to grow in culture and that cell morphology changes characterized by microfilament disorganization is under viral gene control. Further, such cells in culture require the JCV T protein, presumably, to secrete serine proteases into their environment, a characteristic of many human astrocytomas. We have further identified the molecular size of the JCV T protein in productive infections of human glial cells and that successful infection does not require either an association with host p53 proteins or synthesis of the viral small t protein. We have also observed that JCV and a mutant strain of JCV adapted to human kidney cells is able to replicate their DNA and produce virions in astroglial as well as oligodendroglial cells derived from human fetal brain.